The invention relates to augers generally, and more particularly to an auger head assembly for boring rock.
Earth augers are known and typically comprise a cutting head having cutting teeth, and spiral flighting for conveying spoil from the cutting head. However, where hard formations, such as rock, are encountered, drilling rates generally have been limited to 1/2 ft/hr. In addition, these conditions often cause auger damage and breakdown.
In known head assemblies for boring rock, a pair of shanks support rotatably mounted teeth. Among the drawbacks of these assemblies is that the size of the teeth and the retaining mechanism for retaining the teeth in the shank generally necessitate that adjacent teeth be substantially spaced from one other. Those spaces cause ridges to be formed between cutting teeth during excavation. It was believed that the uncut ridges would eventually break off as the head assembly continued to cut. This may be true with a fracturable rock, but when cutting in consolidated rock, such as granite, these ridges of uncut material will stop the auger completely. Even in the case where fracturable rock is encountered, these ridges end up being broken off by the shanks. This causes excessive wear and early failure to the shanks which are much more expensive than the cutting teeth. In addition, the shanks used in these assemblies generally are mirror images of one another. Thus, two teeth, one from each shank, cut along the same path. This configuration does not spread out the downforce of the machine over the entire work surface but only concentrates it on the teeth, thereby adding to increased tooth wear. Further, this configuration can result in excessive vibration causing the teeth to jump. Such vibration also is transferred back to the auger drive where it can loosen fasteners and cause hydraulic motor failure and operator fatigue.
Another problem with current shank design using cylindrical teeth is that the cutting tips of the teeth are at the same height relative to the work and, thus, engage the cutting surface at the same time. When digging in a hard material, such as rock, these teeth are forced to cut their own paths and do not work together to fracture the rock.
Although free rotation of the rotatably mounted drilling teeth in the assemblies described above is important, it often is not achieved when boring hard materials. That is, as the teeth cut into the work surface, the teeth need to rotate in the respective shank in order to undergo uniform wear to maintain their sharpness. Drilling teeth can heat up to over 300.degree. F. when they do not rotate correctly. The heat build-up is caused by the inability of the teeth to rotate which, in turn, causes more friction that exacerbates the problem, e.g., tooth wear. Power requirements also are increased to overcome the friction. As noted above, current shank design generally does not position the teeth for optimal rotation when boring hard materials. This results in the teeth wearing out at excessive rates and penetration rate reduction.
The attack angle at which the center line of the tooth approaches the ground is another factor that relates to the performance of the auger. This angle generally has been limited to a maximum of 45.degree. to avoid adversely affecting tooth rotation. However, this attack angle limitation restricts penetration rates and, thus, increases drilling costs.